As a consequence of increasingly strict legal regulations concerning permissible harmful emissions in motor vehicles which have internal combustion engines, the harmful emissions must be kept as low as possible during operation of the internal combustion engine. On one hand, this can be achieved by reducing the harmful emissions that are produced during the combustion of the air/fuel mixture in the respective cylinder of the internal combustion engine. On the other hand, exhaust-gas postprocessing systems are used in internal combustion engines, converting the harmful emissions that are produced during the combustion process of the air/fuel mixture in the respective cylinder into harmless substances.
Catalytic converters are used for this purpose, converting carbon monoxide, hydrocarbons and nitrogen oxide into harmless substances.
Both selectively influencing the generation of harmful emissions during the combustion, and efficiently converting the harmful components by means of a catalytic converter, require the air/fuel ratio in the respective cylinder to be adjusted very precisely.
The textbook entitled “Handbuch combustion engine”, edited by Richard von Basshuysen and Fred Schafer, 2nd edition, published by Vieweg & Sohn Verlagsgesellschaft mbH, June 2002, pages 559 to 561, discloses a binary lambda control featuring a binary lambda probe which is arranged upstream of the exhaust gas catalytic converter. The binary lambda control comprises a PI regulator, the P- and I-portions being stored in characteristic maps via engine speed and load. In the case of the binary lambda control, the excitation of the catalytic converter, also referred to as lambda fluctuation, is implicitly derived from the on-off control. The amplitude of the lambda fluctuation is set to within approximately 3%.
In order to meet future statutory requirements relating to harmful emissions in particular, use is increasingly made of catalytic converters that are close to the engine. Due to the short mixing section from the outlet valve to the catalytic converter, these often require a very limited tolerance in the air/fuel ratio in the individual cylinders of an exhaust-gas bank, and specifically a significantly more limited tolerance than in the case of a catalytic converter arrangement that is remote from the engine. A cylinder-specific lambda control can be used in this context.
DE 198 46 393 A1 discloses a cylinder-selective control of the air/fuel ratio in a multicylinder combustion engine, featuring a lambda probe which is designed as a jump probe. In the context of said cylinder-selective control, the voltage deviation of the lambda probe voltage signal of a cylinder is formed in relation to the voltage signals of the adjacent cylinders. Correction of the injection is then performed using the difference value. In this case, it is taken into consideration that precisely the distinct change in the probe voltage in the region of the exactly stoichiometric air/fuel ratio allows even small deviations from an optimal air/fuel ratio to be identified.
EP 0 826 100 B1 discloses a method for cylinder-selective control of the fuel/air ratio for an internal combustion engine comprising a plurality of cylinders. Provision is made for a lambda control entity, to which is assigned an oxygen sensor that emits a sensor signal representing a corresponding oxygen content of the total exhaust gas from the individual exhaust-gas packets of the individual cylinders. For each value of the sensor signal, the associated lambda actual value is determined with reference to a characteristic curve. From these values, a lambda mean value is formed for each oxygen sensor, and the difference between a lambda reference value, which is predefined as a function of the load of the internal combustion engine, and the lambda mean value is used as an input variable of a global regulator and is supplied to a global lambda regulator of the lambda control entity for the purpose of correcting the basic injection signal, such that a theoretical air/fuel ratio can be set. Provision is further made for a single-cylinder lambda regulator for controlling the individual air/fuel ratio of the individual cylinders. The cylinder-selective output variable of this single-cylinder lambda regulator is superimposed on the output variable of the global lambda regulator, and a basic injection signal is corrected individually per cylinder using the value that is obtained therefrom.
DE 100 11 690 A1 discloses a cylinder-selective lambda control which features a wideband lambda probe. DE 103 58 988 B3 also discloses a cylinder-specific lambda control in connection with a linear lambda probe.
DE 103 04 245 B3 discloses a method for adapting signal sampling of lambda probe signal values in order to implement a cylinder-selective lambda control for a multicylinder internal combustion engine, wherein time points for capturing the lambda values of the individual cylinders, relative to a crankshaft position of the internal combustion engine, are set such that a characteristic parameter assumes an extreme value which is a measure for the deviation of the lambda values of the individual cylinders.
According to DE 10 2004 026 176 B3, in the context of capturing a cylinder-specific air/fuel ratio for an internal combustion engine, a sampling crankshaft angle is determined relative to a reference position of the piston of the respective cylinder, for the purpose of capturing the measured signal of the exhaust-gas probe, and specifically as a function of a variable which characterizes the air/fuel ratio in the respective cylinder. The measured signal is captured at the sampling crankshaft angle and assigned to the respective cylinder.
DE 10 2004 004 291 B3 discloses capturing the measured signal in an exhaust-gas probe and assigning it to the respective cylinder at a predefined crankshaft angle relative to a reference position of the piston of the respective cylinder. The predefined crankshaft angle is adapted depending on an instability criterion of a regulator. An actuating variable for influencing the air/fuel ratio in the respective cylinder is generated by means of the regulator as a function of the measured signal that is captured for the respective cylinder.
According to DE 10 2005 034 690 B3, a predefined crankshaft angle for capturing an air/fuel ratio by means of a measured signal, for assignment to a respective cylinder, is adapted as a function of a quality criterion that is dependent on irregular running and a driveshaft of the internal combustion engine.